Get the interpolated gradient of height with respect to the terrain at the specified longitude and
latitude relative to the provider's Shape. This method produces a vector representing
the gradient of height in the ShapeFixedFrame.

Note: The direction for the gradient is defined for use as the outward facing surface normal direction.
To obtain the directional derivative for the height (rate of change of height with respect to time),
take the negative gradient and dot it with a unit vector along the desired azimuth (change in height with respect to distance),
times the speed along that direction in meters/second.

The gradient, as defined above, is parallel to the upward facing terrain surface normal,
but its magnitude indicates the rate of change in the height at that location.

Get the interpolated hessian matrix based on the function of height with respect to the specified longitude and
latitude relative to the provider's Shape. This method produces a matrix representing
the mixed second order partial derivatives of height in the ShapeFixedFrame.

Note: The direction for the gradient and hessian is defined for use with the directional derivative on the surface.
The gradient is defined to be orthogonal to the terrain surface with its surface projection in the direction
of the greatest increase in the height. To obtain the second directional derivative for the height
(acceleration of height along a particular azimuth), take the negative of the hessian then pre and post multiply
it by the velocity vector along the desired azimuth. Then add the negative gradient dotted with the derivative
of the velocity vector along the surface.

Loads the terrain data in a given extent into the terrain cache. This method does not return
until the terrain data is fully loaded. All terrain data within the extent is loaded into
memory even if doing so would exceed the TerrainCacheGroup'sMaximumSize. Furthermore, the loaded terrain data is locked
in memory (meaning it will not be unloaded) until the user calls UnloadExtent(CartographicExtent).
This method is safe to call from any thread.

Unloads the cached terrain data for a given extent. This method unloads all cached
regions where any part of the region falls within the extent. It does not return
until the regions have been unloaded. However, if a region within the extent is
being used in another thread while this method is executing, the region may be
reloaded immediately after this method unloads it. This method is safe to call
from any thread.